Select and separate a single branch from your plant and Spray that branch plant for 2-3 weeks from the initiation of flowering to produce pollen sacs on a female plant. Once pollen sacs have formed and are ready, harvest and save the pollen.
Given the plant you have been spraying is female, the pollen produce has no male genes. The pollen you collect is female pollen. When the female pollen is used to fertalise another female plant the seeds you collect will be feminised. The plant grown from these seeds will be female.